1. Field of the Invention
The present invention broadly relates to a vehicle having steerable front and rear wheels (this type of vehicle will be referred to as "4-wheel steering vehicle", hereinunder) and more, particularly, to an apparatus for automatically controlling the steer angle of the rear wheels in such a 4-wheel steering vehicle by controlling an actuator mechanism in response to the operation of a steering wheel for controlling the steer angle of front wheels.
2. Description of the Related Art
A conventional apparatus for controlling the steer angle of rear wheels in a 4-wheel steering vehicle, on which the present invention is based, will be explained hereinunder with specific reference to FIG. 2.
A rotation of a steering wheel 1 caused by the driver is transmitted through a shaft 2 to a gear box 3 and is converted into a linear motion of a linkage 4. The linear motion of the linkage 4 causes a rotation of knuckle arms 5 about their pivot points 5a, thus generating a steer angle .delta.f(t) of the front wheels 6, as a function of time (t). A sensor 31 associated with the shaft 2 senses the angle .delta.h(t) of steering rotation of the steering wheel 1, while a sensor 32 senses lateral acceleration V generated and acting on the vehicle in response to the rotation of the steering wheel 1 by the angle .delta.h(t). Then, a computer 40 operates an actuator 50 in accordance with the signals from the sensors 31, 32, such as to cause a linear motion of a linkage 14 through a gear box 13. The linear motion of the linkage 14 causes knuckle arms 15 to rotate about their pivot points 15a, thus generating a steer angle .delta.r(t) of the rear wheels 16.
The computer 40 determines the rear wheel steer angle .delta.r(t) as being proportional to the lateral acceleration V in accordance with the following formula (1) or, alternatively, by adding the product h.multidot..delta.f(t) of the front wheel steer angle .delta.f(t) and a proportional constant h to the right side of the formula (1) as expressed by the formula (2) below. EQU .delta.r(t)=K.multidot.V (1) EQU .delta.r(t)=h.multidot..delta.f(t)+K.multidot.V (2)
This known steer angle controlling apparatus, however, is not designed to respond to the speed or rate of operation of the steering wheel. In addition, the rear wheels are steered in the same direction as the front wheels regardless of the angle of steering rotation of the steering wheel, so that the turning performance, particularly when the vehicle has to turn sharply, is not so high, although the steering stability during straight running is improved appreciably.
In another known apparatus for controlling the rear wheel steer angle in a 4-wheel steering vehicle, a steering gear for controlling the front wheels and a steering gear for controlling the rear wheels are connected mechanically in such a manner that, when the angle of rotation of the steering wheel is rather small, the rear wheels are steered in the same direction as the front wheels, whereas, when the steering wheel rotation angle is rather large, the rear wheels are steered in the direction counter to the front wheels.
In this apparatus, although the rear wheels are steered in the same direction as the front wheels or in the counter direction to the front wheels depending upon the steering wheel displacement, no consideration is made as to the control of the rear wheel steer angle in response to the speed or rate of operation of the steering wheel by the driver. The driver, when he finds it necessary to quickly steer the vehicle in order to avert from an obstacle or to change the lanes, operates the steering wheel correspondingly quickly but, when he wishes to steer the vehicle along a gentle curve, he operates the steering wheel correspondingly gently. Thus, the driver expects that the vehicle will respond to the rate or speed of operation of the steering wheel. In this known apparatus, however, the rear wheels are steered only in response to the amount of steering wheel displacement so that the vehicle does not delicately respond to the driver's will.
Thus, the above-described conventional 4-wheel steering vehicles suffer from the disadvantage that they fail to meet the driver's request who expects the vehicle to be steered in good response to the speed or rate at which he operates the steering wheel.
In order to overcome these problems, the present inventors have already proposed an apparatus for controlling the steer angle of rear wheels of a vehicle (Japanese patent application No. 102202/1984; U.S. Pat. application Ser. No. 734,332) which is capable of operating in response to the speed of the steering wheel in such a way as to satisfy both the demand for high steering response in the case where a quick turning of the vehicle is required and the demand for high straight running stability in the case where the vehicle is required to turn rather gently. This apparatus is arranged such that the steer angle of the rear wheels of the vehicle is generated in the direction counter to a direction of the steer angle of the front wheels when the steering wheel is operated at high speed, i.e., when the angular frequency of the steering wheel is high, and the steer angle of the rear wheels is generated in the same direction as the steer angle of the front wheels when the steering wheel is operated at low speed, i.e., when the angular frequency is low. According to this apparatus, therefore, when the speed of operation of the steering wheel is high, the apparatus effects control such that the steer angle of the rear wheels is set in the counter direction to the angle of steer of the front wheels. As a result, the steer angle of the front wheels and the steer angle of the rear wheels are set almost simultaneously so that forces are produced simultaneously on the front and rear wheel tires. These forces in turn form yawing moments which act to turn the vehicle in one direction, thus equivalently increasing steering gain, i.e., the ratio of the steer angle of steered wheel to the rotation angle of the steering wheel. Namely, in this case, the sum of the steer angle of the front wheels and the steer angle of the rear wheels forms an effect equivalent to the effect produced by the increased steering gain.
On the other hand, when the speed of operation of the steering wheel is small, control is effected such that the steer angle of the rear wheels is set in the same direction as the steer angle of the front wheels. In consequence, a steering effect equivalent to that produced by a steer angle equal to the difference between the front wheel steer angle and the rear wheel steer angle is obtained, resulting in a small steering gain, and thus improving the steering stability during straight running of the vehicle.
In general, as the speed of a vehicle increases, the sensitivity of turning of the vehicle with respect to the operation of the steering wheel becomes higher, so that when the vehicle is running at high speed, the vehicle may abruptly change its course or posture in response to even a small amount of rotation of the steering wheel. For this reason, when the vehicle is running at an extremely high speed, the rear wheels are preferably steered in the same direction as the steer angle of the front wheels, i.e., in the direction in which the steering gain is decreased and the straight running stability of the vehicle is thereby improved, except for the case where the steering wheel is operated at an abnormally high speed in order to cope with an emergency situation or to avert from an obstacle. On the other hand, when the vehicle is intended to be stopped or is running at an extremely low speed, it is demanded that the posture of the vehicle should be able to be changed by a large margin, i.e., the radius of turning circle of the vehicle should be able to become small, in view of the necessity of changing the course of the vehicle and garaging the same. Accordingly, when the vehicle is running at an extremely low speed, it is preferable to steer the rear wheels in the direction counter to a direction of the steer angle of the front wheels, i.e., in the direction in which the steering gain is increased and the response to the steering input for turning of the vehicle is thereby improved, except for the case where the speed of rotation of the steering wheel is so low as to be close to zero as in the case of effecting a normal circular turn.